1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and a method for driving the apparatus. More particularly, the invention relates to a driving circuit adapted advantageously to a liquid crystal display apparatus utilizing anti-ferroelectric liquid crystal as its liquid crystal material, and a method for driving the LCD apparatus.
2. Description of the Related Art
Anti-ferroelectric liquid crystal (AFLC) is one of a variety of liquid crystal materials used by liquid crystal displays (LCD). An LCD apparatus that utilizes AFLC causes liquid crystal molecules to block or transmit light therethrough by driving the molecules between two phases: an anti-ferroelectric phase with no electric field turned on, and a ferroelectric phase with an electric field activated.
Threshold-less anti-ferroelectric liquid crystal (TL-AFLC) is particularly noted for its wide angles of visibility and its high response speed. A voltage-transmittance curve (V-T curve) of TL-AFLC has a symmetrical V-shaped characteristic around an origin, as shown in FIG. 7. In terms of response times, twisted nematic (TN) liquid crystal takes dozens of milliseconds to respond when driven, while TL-AFLC merely takes tens of microseconds to act. That means TL-AFLC is quicker to respond than TN liquid crystal by as many as three orders of magnitude.
Alternate driving is one of the commonly employed methods for driving LCD. The alternate driving method involves driving LCD by alternating, illustratively in increments of frames, the polarity of video signals (voltages) applied to the liquid crystal through the use of an AC voltage. Generally, one frame lasts about 16 milliseconds. Within that time frame, each scanning line is fed with a gate pulse whose width is about 16 microseconds needed to drive all scanning lines of the frame illustratively on an XGA display. The pulse width varies with the number of scanning lines used.
One disadvantage of the conventional alternate driving method above is that when applied to an LCD apparatus using TL-AFLC as its liquid crystal material, the method causes the LCD to lose its response speed resulting in a moving picture afterimage phenomenon. The reason is as follows: if the width of a gate pulse applied to each scanning line is illustratively 16 microseconds, that means the write time per scanning line is 16 microseconds as opposed to the response time of tens of microseconds of TL-AFLC. When the response time of TL-AFLC is longer than its write time, the writing of data within a single frame time fails to let TL-AFLC fully respond. Hence the inability to obtain an adequate transmittance.
To obtain the necessary transmittance requires writing data not in a single frame but across multiple frames. This translates into an effectively prolonged response time for the LCD as a whole. For example, the writing of data in five frames involves a response time of 16 millisecondsxc3x975=80 milliseconds. That is, the response time of TL-AFLC ends up being equivalent to that of TN-LCD. In order to suppress moving picture afterimages, it is necessary ideally to complete the writing of data within a single frame time. With the conventional driving method, however, afterimages are unavoidable because the method requires carrying out data writes over a plurality of frames. Although TL-AFLC used in the LCD is supposed to offer a high response speed, the actual response speed of the liquid crystal material when driven is reduced to levels of other slow-to-respond liquid crystals. There has been a need for methods that would make full use of the inherently quick response of TL-AFLC used in LCD apparatuses.
The present invention has been made in view of the above circumstances and provides a liquid crystal display apparatus having a driving circuit to make full use of the inherently high response speed of TL-AFLC, and a method for driving the apparatus.
In carrying out the invention and according to a first aspect thereof, there is provided a liquid crystal display apparatus having a driving circuit comprising: a signal line driving means having anti-ferroelectric liquid crystal sandwiched between an active matrix substrate and an opposed substrate, the active matrix substrate having a plurality of signal lines and a plurality of scanning lines arrayed in a matrix fashion to constitute a plurality of pixels, the signal line driving means driving the plurality of signal lines; a scanning line driving means for driving the plurality of scanning lines; and a reset voltage applying means for use when video signals are written to all pixels on any one of the plurality of scanning lines in one horizontal period, the reset voltage applying means applying a reset voltage for resetting beforehand any voltages remaining in all pixels on a plurality of scanning lines which are contiguous to the one scanning line and to which the video signals are written following the one horizontal period, the applying of the reset voltage being performed prior to the one horizontal period in which to write the video signals to all pixels on the one scanning line and over a plurality of horizontal periods temporally continuous to the one horizontal period.
According to a second aspect of the invention, there is provided a liquid crystal display apparatus having a driving circuit comprising: a signal line driving means having anti-ferroelectric liquid crystal sandwiched between an active matrix substrate and an opposed substrate, the active matrix substrate having a plurality of signal lines and a plurality of scanning lines arrayed in a matrix fashion to constitute a plurality of pixels, the signal line driving means driving the plurality of signal lines; a scanning line driving means for driving the plurality of scanning lines; and a reset voltage applying means for use when video signals are written to all pixels on any one of the plurality of scanning lines in one horizontal period, the reset voltage applying means applying a reset voltage for resetting beforehand any voltages remaining in all pixels on a plurality of scanning lines which are separated from the one scanning line and to which the video signals are written following the one horizontal period, the applying of the reset voltage being performed prior to the one horizontal period in which to write the video signals to all pixels on the one scanning line and over a plurality of horizontal periods temporally separated from the one horizontal period.
There are liquid crystal display apparatuses that adopt what is known as a line sequential driving method whereby scanning lines are sequentially scanned from top to bottom while being fed with a signal one line at a time. For such LCD apparatuses, one frame time divided by the number of scanning lines gives a driving time per scanning line (for one horizontal (1H) period). That is, there is no spare time left in each frame time. On the other hand, writing a signal to the signal line driving means (source driver) leaves enough time to spare. A clock signal for the signal line driving means generally contains per horizontal period the pluses numbering greater than the number of signal lines. Upon completion of the writing of data corresponding to the actual number of signal lines, a short time (e.g., equivalent to about 10% of the total number of pulses in each horizontal period) is left out as a blanking period.
Taking account of such a spare time existing per horizontal period upon writing of a signal to the signal line driving means, the inventors of this invention came up with an idea: that any voltage remaining in the liquid crystal should be initially reset by taking advantage of the spare time, followed by a write operation (i.e. application of a voltage) in order to let the liquid crystal fully respond to the subsequently applied voltage. The term xe2x80x9cresetxe2x80x9d refers herein to a state where no voltage is being applied to the liquid crystal. In other words, the reset voltage is a zero voltage.
Because it also takes time for the liquid crystal to respond when its energized state is reset to a zero voltage state, the spare time within each horizontal period is not sufficient as a time in which to apply the reset voltage; a complete reset state is yet to be reached. This bottleneck is circumvented by the driving circuit of the liquid crystal display apparatus according to the first aspect of the invention as follows: when video signals are written to all pixels on any one of the plurality of scanning lines in one horizontal period, the driving circuit applies a reset voltage for resetting beforehand any voltages remaining in all pixels on a plurality of scanning lines which are contiguous to the one scanning line and to which the video signals are written following the one horizontal period. Application of the reset voltage takes place prior to the one horizontal period in which to write the video signals to all pixels on the one scanning line and over a plurality of horizontal periods temporally continuous to the one horizontal period. According to the second aspect of the invention, the driving circuit of the liquid crystal display apparatus applies a reset voltage for resetting beforehand any voltages remaining in all pixels on a plurality of scanning lines which are separated from the one scanning line and to which the video signals are written following the one horizontal period. Application of the reset voltage takes place prior to the one horizontal period in which to write the video signals to all pixels on the one scanning line and over a plurality of horizontal periods temporally separated from the one horizontal period.
In any case, although the reset voltage application time within each horizontal period is short, applying the reset voltage over a plurality of horizontal periods makes it possible to reach a complete reset state. After the complete reset state has been attained, the writing of data to each pixel starts from a zero voltage state in the positive or negative voltage direction, which shortens the response time of the liquid crystal. Referring to FIG. 7, inversion of the applied voltage from +V1 to xe2x88x92V1 by the conventional driving method entails a prolonged response time because the liquid crystal responds by tracing a V-shaped path indicated by arrows Y1 and Y2. The inventive scheme, by contrast, applies the voltage starting from 0 V following a reset. In this case, the liquid crystal need only trace half of the V-shaped path indicated by arrow Y2, which means the conventionally experienced response time is approximately halved.
When a reset voltage is applied to a plurality of scanning lines, the number of scanning lines to which to apply the reset voltage simultaneously is preferably an integer multiple of xcfx84 off/xcfx84 reset, where xcfx84 of f is the longest of graduated response speed fall times and xcfx84 reset is the time in which to apply the reset voltage. A maximum allowable number of scanning lines to which to apply the reset voltage simultaneously should be one which corresponds to a half frame. One disadvantage of a scanning line count exceeding the half frame is that users will have difficulty perceiving the continuity of pictures and find screens inordinately darkened.
In the liquid crystal display apparatus according to the second aspect of the invention, a sum of a reset time and a wait time is preferably half of one frame time at most, the reset time ranging from the time when the reset voltage applying means starts applying the reset voltage to all pixels on one scanning line, to the time when the applying of the reset voltage ends, the wait time ranging from the time when the applying of the reset voltage to all pixels on the one scanning line ends, to the time when the writing of the video signals starts. That is, when the reset voltage is applied to a plurality of scanning lines which are separated from the scanning line subject to a write operation, as in the second aspect of the invention, there is a rule of thumb for the separation whose extent must not be arbitrary. Since applying the reset voltage to scanning lines erases displays of all pixels thereon, if the sum of the reset time and the wait time exceeds half the frame time, users will have difficulty perceiving the continuity of pictures and find screens inordinately darkened.
According to a third aspect of the invention, there is provided a method for driving a liquid crystal display having anti-ferroelectric liquid crystal sandwiched between an active matrix substrate and an opposed substrate, the active matrix substrate having a plurality of signal lines and a plurality of scanning lines arrayed in a matrix fashion to constitute a plurality of pixels, the method comprising the steps of: when video signals are written to all pixels on any one of the plurality of scanning lines in one horizontal period, applying a reset voltage for resetting beforehand any voltages remaining in all pixels on a plurality of scanning lines to which the video signals are written following the one horizontal period, the applying of the reset voltage being performed over a plurality of horizontal periods prior to the one horizontal period; and applying, to all pixels on one scanning line fed with the reset voltage, a driving voltage at least 1.5 times a graduated voltage determined by the liquid crystal material in use in order to write the video signals.
As outlined above, the liquid crystal display apparatus according to the invention shortens the response time of the liquid crystal by use of the driving circuit. Still, the shortening of the response time can be insufficient depending on the type of liquid crystal or under various conditions specific to the LCD apparatus in question. That is, the writing of video signals may not be accomplished within a single frame. In such a case, the write voltage may be raised to reduce the response time. The response time reduction is made possible because the response time xcfx84 of liquid crystal is subject to the relation
xcfx84xe2x88x9d1/(Psxc2x7E)xe2x80x83xe2x80x83(1)
where, Ps denotes spontaneous polarization of the liquid crystal and E denotes an applied electric field.
For a liquid crystal display apparatus, its V-T curve (voltage-transmittance characteristic curve) is used as a basis for establishing graduated voltages in keeping with a desired gradation count. Since the V-T curve varies from one liquid crystal material to another, the graduated voltages are determined specifically with respect to the liquid crystal material used by the LCD apparatus in question.
The relation (1) above may be modified as follows:
xcfx84xe2x88x9dd/(Psxc2x7V)xe2x80x83xe2x80x83(2)
where, d represents a substrate-to-substrate gap (i.e., thickness of the liquid crystal layer) and V denotes the applied voltage. From the relation (2) above, it will be appreciated that the response time is also shortened by reducing the substrate-to-substrate gap.
Other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings.